As a prior-art crystallization device, such a device is known in which a draft tube b is provided at a center part in a sealed vertical container a, a stirring blade c is pivotally supported on a lower part in the draft tube b, a baffle e is suspended in a swollen portion d in a lower half part of the vertical container a with an inside in the swollen portion d outside the baffle e being formed as a set ring region f, a classification leg g is provided on a lower part of the vertical container a, a crystal outflow tube h is connected to the classification leg g, a circulation pipeline i is connected between a lower end part of the classification leg g and the swollen portion d with a pump j interposed in the circulation pipeline i, the stirring blade c is rotated so as to circulate a suspension slurry inside and out of the draft tube b, the crystal having grown to a desired size during this circulation goes out of a circulation flow and settles on a bottom part in the vertical container a, the crystal classified in the classification leg g flows out of the outflow tube h, and a part of a solution containing microcrystals in the set ring region f flows to the classification leg g via the circulation pipeline i, while another part thereof is heated, whereby the microcrystals are dissolved, and is returned into the container a (see Non-Patent Literature 1).